Field of the Invention
The present invention generally relates to radial magnetic couplings that may be used in rotary equipment, such as pumps, mixers and compressors, and more particularly to rotary devices having a radial magnetic coupling wherein the magnetic coupling transmits torque from one component to another through a stationary fluid barrier.
Discussion of the Prior Art
In many rotary devices, such as pumps, mixers and compressors, it is desirable to avoid rotating seals. Magnetic couplings have been developed with a magnet coupling that utilizes a driven component and a drive component with the driven component being driven via a non-contacting permanent magnet coupling in a radial magnet orientation. Such equipment frequently is referred to as being sealless, but actually includes inner and outer magnets separated by a canister that is sealed with one or more static seals.
Radial magnetic couplings that utilize permanent magnets are common, for example, in rotodynamic (also known as kinetic or centrifugal) pumps. The radial magnetic couplings consist of three main components: a larger, outer coupling component (also known as an outer magnet ring or outer rotor) with multiple permanent magnets on its inner surface; a smaller, inner coupling component (also known as an inner magnet ring or inner rotor) with multiple permanent magnets on its outer surface; and a containment canister (also known as a can, shell, shroud, barrier or portion of a casing) separating the inner and outer components and forming a stationary boundary or barrier for the fluid chamber. The magnets on the inner and outer components are disposed in axial alignment with each other to match up and synchronize the inner and outer components, such that as one component is rotated, the other component is synchronized and forced to follow, whereby the pump impeller or pumping rotor is driven. Neither of the inner or outer coupling components physically touches the other, and they rotate in separate environments, separated by the canister.
The radial magnetic couplings are of two configurations, “outer drive” and “inner drive”. Most radial magnetic couplings have an outer drive arrangement in which the outer magnetic coupling component is outside of the canister, and therefore, outside of the equipment's fluid chamber, which would be contained within the canister. The outer magnetic coupling component in such equipment usually is driven by an external power source, such as a motor. In such configurations, the inner magnetic coupling component is disposed inside the equipment's fluid chamber and is connected to the equipment's rotor. Thus, in such outer drive arrangements, a containment canister provides the boundary for the equipment's fluid chamber, with the fluid chamber being inside the canister.
Although less common, some radial magnetic couplings have an inner drive arrangement, which utilizes the same three general components, but the roles are reversed. The inner magnetic coupling component is inside of the canister, and the equipment's fluid chamber is defined by the space between the outside of the canister and the casing. The inner drive portion also usually is driven by an external power source, such as a motor, while the outer magnetic coupling component is outside of the canister and is within the equipment's fluid chamber, and is connected to the equipment's rotor. The canister again provides the boundary for the equipment's fluid chamber, but in an inner drive arrangement the fluid chamber is outside of the canister.
Permanent magnet coupled pumps generally utilize end suction via an axial inlet, are of single stage or multistage configuration, and may include an overhung impeller design. The overhung impeller design has the impeller mounted on a rotor assembly which contains a first magnet ring of a magnet coupled drive spaced from the pumping element. A second magnet ring is mounted on the rotatable shaft of a frame that is coupled to a motor or power drive device. The pump, frame that supports the rotatable shaft and the power drive device generally are mounted on a common base plate. Close coupled permanent magnet coupled pumps tend to be of a somewhat similar construction to the separately coupled version, except that the second magnet ring is mounted directly on the driver shaft of the power drive device. The drive section utilizes permanent magnets or an eddy current drive system to transmit power to the impeller. This type of sealless pump uses a standard motor to drive the second magnet ring, which in turn, via the magnetic coupling, drives the first magnet ring. A containment canister that contains the process fluid sealingly separates the magnet components.
Typically, the canister is a unitary (1-piece) design shaped like a cup, with a thin generally cylindrical portion between the magnets and a flat or domed portion closing off one end. The thin cylindrical portion is advantageous to minimize the total radial gap between the inner and outer magnets, so as to create more torque for a given volume of magnet material. In most cases, the canister is made of metal, since metal allows for a strong yet thin design. But because metal is electrically conductive, eddy currents are created in the canister when the coupling is rotated due to the rotating magnetic field between the inner and outer magnets. These eddy currents convert some of the transmitted power into heat, which wastes power and often has detrimental effects on the equipment and/or the fluid within the equipment.
Some canister designs use non-metallic composites instead of metal. This eliminates the eddy current heating, but usually results in a much thicker cylindrical portion between the magnets, which increases the total radial gap between the magnets and therefore reduces the torque created for a given volume of magnet material.
Most magnetic coupling designs also require the magnets to be protected from contact with the fluid inside the equipment. For a rotor that is inside the fluid chamber, this usually requires a separate component or components, such as a sleeve or coating between the magnets and the canister, which unfortunately increases the total radial gap between the magnets, and therefore, reduces torque.
All magnetic coupling designs have some form of a bearing system to support a rotor inside the fluid-chamber. This support is normally needed in both a radial and an axial direction. In most cases, the bearing system is a plain bearing system, where the support is accomplished by bearing surfaces sliding against each other. In most cases, these systems have separate stationary and rotating components, each with one or more bearing surfaces that engage each other to provide radial or axial support. When the sole or primary purpose of one of these components is to provide a bearing surface, then the component is commonly referred to as a bushing. The bushing components usually are not positioned within the radial gap between the magnets, since this would greatly increase the total radial gap between the magnets, and therefore, greatly reduce torque. Instead, they commonly are positioned on the rotor beyond one end or both ends of the magnets.
When a bushing is positioned at only one end, the rotor support is cantilevered, which disadvantageously allows much more deflection of the rotor. When bushings are positioned beyond both ends of the magnets, the undesired cantilever support is eliminated, but special care must be taken to ensure the bushings are precisely aligned with each other and the equipment may require greater axial space.
Also, it generally is advantageous to have a full fluid film between the bearing surfaces of the rotating bushing and the stationary component, especially when the bushing is of an elongated design, because it reduces friction and wear. For radial bushings, one way to improve the fluid film is to make the bearing surface of the bushing axially longer. However, because the bushings typically are positioned beyond either end of the magnets, lengthening the bearing surfaces would disadvantageously require substantially greater axial space.
The present disclosure addresses shortcomings in prior art equipment having a magnetic coupling, while providing numerous advantages over the prior art, as discussed herein.